It's being powered by 5v from the arduino board (BBB, Modern Devices).

Once and awhile I can faintly hear the servo kind of "hiccup" (just a slight bzzzzzt lasting a fraction of a second with no perceivable movement in the servo horn). I have a feeling the signal line is picking up some interference? Would a pull-down resistor on the signal line solve this (10k?)? My gut tells me though the servo's electronic package already pulls the signal line to ground though so I'm not sure.

I doubt that a pull down would make any difference, I think you have another problem but there is not enough information supplied to hazard a guess.If you want to post the code and a schematic theat might be something some one can spot.

From my experience, I would also think it's a power supply problem, and not a signalproblem. The power leads should be short and heavy. Also, I always put bypass [100 nF] and electrolytic caps [100-220 uF] across the servo power buss to help filter noise.

All in all, you should probably try different things to get more info. Eg, start with tackinga pulldown on the signal wire, and see if that helps, on and on.

Are you doing anything else in the code besides updating the servos? Specifically anything which uses interrupts or disables them? I've found that when the timing of the pulses that goes to the servos isn't perfect the servo will do that since it thinks it's slightly off from there it should be.